1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device configured by a thin film transistor. Particularly, the present invention relates to a method of fabricating a semiconductor device having a plurality of circuits driven by providing different voltages of electric sources. Moreover, the present invention relates to an electronic device using the foregoing semiconductor device.
2. Related Art
In recent years, the development of a semiconductor device having a circuit formed with a thin film transistor (hereinafter, referred to as TFT), which is configured using a semiconductor thin film formed on the substrate having an insulation surface has been proceeded. As an representative example of a semiconductor device having a circuit formed with a TFT, an active matrix type liquid crystal display device, an active matrix type OLED (Organic Light Emitting Diode) and the like are known. Now, a method of fabricating a TFT will be exemplified below. It will be described below with reference to FIG. 9.
As shown in FIG. 9A, a polycrystal semiconductor film prepared by a method of crystallizing an amorphous semiconductor film or the like on a substrate 1101 having an insulation surface is subjected to the patterning, thereby a semiconductor active layers 1102c and 1102d being formed. On the semiconductor active layers 1102c and 1102d, an insulating film 1103, an electrically conductive film 1104 and a resist 1186 are formed. Since a gate electrode of a TFT is formed with the electrically conductive film 1104, it is defined that the electrically conductive film 1104 is also referred to as a gate metal. It should be noted that in FIG. 9, an example in which a gate metal is formed in a mono-layer structure is shown.
When the resist 1186 has been formed, a resist mask for performing the patterning of a gate metal is fabricated. The resist 1186 is exposed to light through the pattern, and the resist 1186 is photosensitized. Subsequently, by developing it, masks (resist masks) 1123, 1124, as shown in FIG. 9B, consisted of a resist are formed. The electrically conductive film 1104 is etched using the resist masks 1223, 1224. Thus, a gate electrode 1121 and a gate electrode 1122 are fabricated. Subsequently, an impurity element which gives the N-type impurity is doped (doping 1). In this way, the N-type impurity regions 1125a, 1125b, 1126a and 1226b are formed within the semiconductor active layers 1102c, 1102d. 
Subsequently, as shown in FIG. 9C, after the resist masks 1123, 1124 have been removed, a resist mask 1128 is newly formed. Subsequently, an impurity element which gives the P-type impurity is doped (doping 2). Thus, impurity regions 1129a, 1129b are formed within the semiconductor active layer 1102d. Here, in the impurity regions 1129a, 1129b, the N-type impurity has been added in the doping 1. However, the impurity regions 1129a, 1129b functions as a source region and a drain region of the P-channel type TFT without any problem by adding the P-type impurity element in a high concentration in the doping 2.
In this way, the N-channel type TFT and the P-channel type TFT can be formed.
In recent years, the properties such as electric field effect mobility of TFT in which a crystalline semiconductor film (typically, polycrystal film) (hereinafter, referred to as polycrystal TFT) is made an active layer, or the like have been enhanced. Therefore, it becomes also capable of forming a circuit equipped with a variety of functions using the relevant TFT. Hence, it has been expected that the circuit conventionally fabricated on a single crystal substrate is formed on a substrate having an insulation surface such as a glass substrate or the like using TFT, and the attempts have been performed. For example, it has been expected that an arithmetic processing circuit, a memory element and the like are formed using a TFT on a substrate which is the same with the substrate on which a pixel of a display device of liquid crystal display device and the like have been formed.
Now, in the case where a variety of circuits are formed using a TFT on the same substrate having an insulation surface, the properties required to the TFT configuring the relevant circuits are different according to the functions of the respective circuits. Therefore, it is necessary to differently make TFTs having different properties. Hereinafter, the difference of the properties required to TFTs configuring the relevant circuits according to the functions of the circuits will be described by exemplifying the concrete examples.
For example, the case where an active matrix type liquid crystal display device and an arithmetic processing circuit are formed on the same substrate using a TFT is exemplified. An active matrix type liquid crystal display device has a pixel section configured with a plurality of pixels disposed in a matrix shape and a drive circuit section for inputting a picture signal to the foregoing pixel section (hereinafter, referred to as pixel drive circuit section).
In FIG. 12, one example of the configuration of a pixel section of an active matrix type liquid crystal display device is shown. In the pixel section, a plurality of signal lines S1-Sx and scanning lines G1-Gy are disposed. At each intersection of the signal lines S1-Sx and the scanning lines G1-Gy, a pixel is disposed. Each pixel has a switching element. The foregoing switching element selects an input to each pixel of the picture signal inputted into the signal lines S1-Sx according to the signal inputted into the scanning lines G1-Gy. In FIG. 12, a TFT 3002 (hereinafter, referred to as pixel TFT) is shown as the foregoing switching element. Moreover, each pixel has a holding capacity 3001 for holding a signal inputted into a pixel from the signal lines S1-Sx and a liquid crystal element 3003 whose transmittance is changed according to the picture signal via the pixel TFT 3002.
At each pixel, a gate electrode of the pixel TFT 3002 is connected to one of the scanning lines G1-Gy. One of the source region or the drain region of the pixel TFT 3002 is connected to one of the signal lines S1-Sx, the other is connected to one of the electrodes of the holding capacity 3001 and one of the electrodes of the liquid crystal element 3003.
The pixel TFT 3002 configuring a pixel is required for the off-state current being slight. It is for the purpose of preventing the voltage applied between the electrodes of the liquid crystal element 3003 disposed at each pixel from being changed, the transmittance from being changed, and the image from being disturbed. Moreover, in an image visible recognition via the pixel TFT 3002 type (hereinafter, referred to as transmitting type) liquid crystal display device, in order to enhance the aperture opening ratio, it is required to make the pixel TFT 3002 refined. Furthermore, between the electrodes of the liquid crystal element 3003, usually, the voltage of about 16V is applied. Therefore, the pixel TFT 3002 or the like is required to withstand the voltage of about 16V. Hence, it is desirable it is configured so that a TFT has a low concentration impurity region overlapped with the gate electrode (hereinafter, referred to as Lov region) and a low concentration impurity region not overlapped with the gate electrode (hereinafter, referred to as Loff region) in its structure.
On the other hand, a TFT configuring a pixel drive circuit section (hereinafter, referred to as TFT for pixel drive circuit) is not required to reduce the off-state current and make it refined as the pixel TFT is required. However, since it operates by an electric source voltage of about 16V, it is required to withstand the voltage.
In an arithmetic processing circuit, a high drive frequency is required. Therefore, a TFT configuring the arithmetic processing circuit is required to enhance the mobility of carrier and be refined. On the other hand, an arithmetic processing circuit fabricated by a refined TFT is capable of being operated by about 3-5 V of an electric source voltage, and the withstanding voltage of the TFT is not so much required as those of a pixel TFT and a TFT for pixel drive circuit are required.
It is necessary to differently make TFTs according to the above-described required properties.
Hence, an object of the present invention is to provide a method of fabricating a semiconductor device, which is capable of differently making a plurality of kinds of TFTs that have different properties respectively or are different in design rule on the same substrate.